1. Technical Field
The present invention relates to a piezoelectric drive device and various apparatuses such as a robot including a piezoelectric drive device.
2. Related Art
A piezoelectric actuator (piezoelectric drive device) which drives a driven body by vibrating a piezoelectric substance is used in various fields, since a magnet or a coil is not required (for example, JP-A-2004-320979). According to a basic configuration of the piezoelectric drive device, four piezoelectric elements are configured to be arranged in two rows and two columns on two surfaces of a reinforcing plate. Eight piezoelectric elements in total are disposed on both sides of the reinforcing plate. Each of the piezoelectric elements is a unit which interposes each piezoelectric substance between two electrodes. The reinforcing plate is used as one electrode of the piezoelectric element. One end of the reinforcing plate has a protrusion portion disposed in order to rotate a rotor by coming into contact with the rotor serving as a driven body. If an AC voltage is applied to the two piezoelectric elements arranged at opposite angles among the four piezoelectric elements, the two piezoelectric elements perform expansion and contraction movement. In response to the movement, the protrusion portion of the reinforcing plate performs reciprocating movement or elliptical movement. Then, in response to the reciprocating movement or the elliptical movement of the protrusion portion of the reinforcing plate, the rotor serving as the driven body is rotated in a predetermined rotation direction. The rotor can be rotated in the opposite direction by switching from the two piezoelectric elements to which the AC voltage is applied to the other two piezoelectric elements.
An output Pw of a piezoelectric drive device is proportional to the product of drive force F and drive speed v of the piezoelectric drive device. Here, the drive force F of the piezoelectric drive device is proportional to a cross-sectional area of a piezoelectric element (specifically, piezoelectric substance interposed between electrodes) configuring the piezoelectric drive device. Therefore, according to a dimensional analysis, if a length dimension representing a dimension of the piezoelectric element is indicated by [L], the drive force F is proportional to [L]2. The drive speed v of the piezoelectric drive device is proportional to a length [L]1 of the piezoelectric element, and a resonance frequency fr is proportional to [L]−1. Accordingly, the drive speed v has no relationship with [L]. Therefore, it is considered that the output Pw of the piezoelectric drive device is proportional to [L]2. The output Pw of the piezoelectric drive device can be increased simply by increasing the dimensions of the piezoelectric element configuring the piezoelectric drive device and increasing the volume of the piezoelectric element.
However, a weight Wt of the piezoelectric drive device is proportional to a volume [L]3 of the piezoelectric element. Accordingly, a power-to-weight ratio Pw/Wt (ratio of the output Pw to the weight Wt) of the piezoelectric drive device becomes proportional to [L]−1. Consequently, a problem arises in that simply increasing the dimension of the piezoelectric element and increasing the output Pw of the piezoelectric drive device may unreasonably cause a decrease in the power-to-weight ratio Pw/Wt. For this reason, a technology has been desired which can provide a lightweight piezoelectric drive device having high output while the decrease in the power-to-weight ratio Pw/Wt is prevented. In addition, for the piezoelectric drive device, miniaturization, low cost, resource saving, easy manufacturing, and improved usability have been desired.